1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus and a recording and reproducing method thereof, for allowing information light and reference light to interfere with each other in an optical recording medium, and recording the pattern of a fringe thereof on an optical recording medium as information, and irradiating the optical recording medium with reference light to reproduce the information.
2. Description of the Related Art
An optical recording medium has established itself as a recording medium familiar to users due to its characteristics such as portability and non-contact property, and recently, the market for DVDs as recording media of movies and music is growing.
However, the recording capacity required in an optical recording medium is greatly increasing along with the further enhanced quality of picture and music, and a method of enhancing a recording density is actively being studied and developed. Above all, recently, a volume-type high-density optical information recording system using holography is drawing attention. In the optical information recording system using holography, information light and reference light are allowed to interfere with each other, and a three-dimensional fringe thus formed is recorded. According to this system, during reproduction, the recorded fringe is irradiated with reproduction light in the same direction as that of the reference light, whereby information is reproduced by interference. According to such a holography system, a recording medium is allowed to have information even in a thickness direction, so that a recording density can be enhanced remarkably compared with the conventional optical information recording system.
As the optical information recording system using holography, a two-beam interference system for irradiating a recording medium with information light and reference light from two separate optical paths as well as a collinear system for aligning information light and reference light coaxially and irradiating an optical information recording medium with those lights are known. Regarding the collinear system, the detail thereof is described in Tan Shochi and one other, “Holographic Memory/Measurement and Nano-control Technology Supporting HDVTM”, (Proceeding of 35th Meeting on Lightwave Sensing Technology), June 2005, pp. 75-82.
Among those systems, the collinear system is advantageous in that a conventional optical system of an optical disk can be applied without a substantial change. Hitherto, there are a number of practical proposals for the optical information recording system using a hologram of a collinear system.
For example, Japanese Patent Application Laid-open No. H11-311936 discloses the use of a conventional optical information recording system that rotates a recording medium so as to attain further practicability. According to this system, as shown in FIG. 5, an optical recording medium 46 includes a layer (hereinafter, referred to as an address layer) 41 having address information and a guide groove, and a layer (hereinafter, referred to as a hologram layer) 42 for recording information using holography provided separately from the address layer 41. With such the configuration, a hologram 44 is formed in the optical recording medium 46 with recording and reproduction laser light 43 while tracking with servo laser light. Then, the hologram 44 is recorded in the hologram layer 42 in the optical recording medium 46.
Further, the information recorded in the hologram layer 42 in the optical recording medium 46 can be read with the recording and reproduction laser light 43, and the compatibility with the conventional optical information recording and reproducing apparatus is also ensured.
However, in such a configuration, the servo laser light for tracking or the like is irradiated at the same time as the laser light for recording and reproducing information.
For example, during reproduction, the servo laser light is overlapped as stray light with reproduction laser light carrying information recorded in the hologram, so that there is a problem that the quality of a signal is impaired. In the conventional optical recording medium 46 shown in FIG. 5, an attempt is made to suppress the influence by stray light by inserting a dichroic mirror layer 45 between the address layer 41 and the hologram layer 42. However, the effect thereof is limited since it is difficult to completely separate each laser light.
Further, the simultaneous irradiation of two laser light beams during recording causes a problem in terms of the power consumption of an apparatus.